The present invention relates to a method for opening a nip in an extended-nip press and an apparatus for opening a nip in an extended-nip press.
In an extended-nip press, formed in part by a back-up roll, the back-up roll is usually heated and a press felt and glide-belt mantle which run through the extended nip may be damaged if the press machine has to be stopped from some reason because of a disturbance in the running of the web through the paper machine. For example, when the temperature of the water in a hot-water heated back-up roll is about 140.degree. C. and the surface temperature of the back-up roll is about 80.degree. C. during running of the press machine, after stopping the press machine, the surface temperature of the outer surface of the back-up roll rises to about 120.degree. C. At this high temperature, it is drawback that prolonged exposure of the press felt and glide-belt mantle in the vicinity of the heated surface of the back-up roll may damage the press felt and glide-belt mantle.
Another drawback of existing extended-nip presses is that since the compression of the back-up roll against the closed glide-belt mantle is generally from about 20 mm to about 35 mm, contact between the glide-belt mantle and the back-up roll cannot be removed even if the glide shoe is pulled into the open position. It is also not possible to displace the back-up roll and the extended-nip roll rapidly into an open position in relation to one another, because, owing to the very high maximal load, the press force is transferred through draw bars or through the frame skeleton.